The research aims to establish the molecular basis of calcium absorption across the small intestine and the mechanism of regulation by vitamin D, dietary calcium, growth and aging, pregnancy and other factors. The absorption involves an active cation pump for calcium and comprises two or more steps, including entry at the mucosal surface and energy-coupled exit at the serosal face. Vitamin D maintains tissue components required for the transport. The identification and characterization of these is the main focus of this proposal. Recently we have identified and partially characterized a vitamin D-dependent, particulate, calcium-binding activity of high molecular weight which correlates with the calcium transport and is believed to be a membrane component of the translocation mechanism. This calcium-binding complex (CaBC) has been solubilized and contains at least 3 vitamin D-dependent activities: calcium binding with high affinity; p-nitrophenylphosphatase and Ca-dependent ATPase. Column chromatography has separated the first from the last two activities. Further isolation and reconstitution studies are in progress to characterize the molecular basis of vitamin D-dependent calcium absorption.